Primitive Text Functions for the HTML5 <canvas> Element

The <canvas> element provides the browser's javascript with a two dimensional drawing area. It is a simple path/stroke-fill model but provides the primitives necessary for making sophisticated graphical presentations. And you will need to be sophisticated with your grpahical presentation because you won't be using any printed words or numbers. What could have been the holy grail of javascript to user communication was turned into a mime by the omission of text.

Fortunately the 1960s has come to the rescue. A. V. Hershey created a set of simple vector fonts for pen plotters which were released by the US National Institute of Standards (NIST). The code presented here encodes the printable 7-bit ASCII characters in a javascript file with a handful of functions to add text operations to the canvas element.

The Code

You can find the code at, or in your browser cache since it got loaded with this page. As written it is about 14k bytes. If you ran it through a javascript minimizer and prezipped it you would squeak it into two 1500 byte datagrams. Allow me to stress this: Do not link to this file. I will mess you up if you try to save bandwidth by using my copy in production.

The demonstration code that is drawing that little box on the right is found in


This code is released to the public domain. Do with it as you will.


In a nutshell the following methods are added to your canvas context:
ctx.drawText = function(font,size,x,y,text)
Draw text in the specified font and size at the position (baseline). Font is ignored for now, there is only one. The width of the line is set automatically from the font size. The ctx.strokeStyle is used.
ctx.measureText = function(font,size,text)
Return the width of the text if it were drawn with the current settings.
ctx.fontAscent = function(font,size)
This is the ascent from the baseline. It is taller than the capital letters. It sort of encompasses the ascent and half the leading.
ctx.fontDescent = function(font,size)
ctx.drawTextRight = function(font,size,x,y,text)
Just for lazy programmers.
ctx.drawTextCenter = function(font,size,x,y,text)
Also for lazy programmers.

Future Work

The bodies responsible for defining the <canvas> should spend a day or two and define some text functions. Yes, I realize we all have different fonts and even within allegedly same fonts the metrics are different. We are programmers. We will deal with that. Give us three pseudo named fonts, "default", "sans", and "serif" perhaps and then let us use the style sheet for anything else the graphics designers want to inflict on the users.

More narrowly...

Related Work

Rendering text with <canvas>
The CanvasPaint folks do a nice survey of text rendering options.
Rhino Canvas drawString
A proposed model for text in canvas.
Textout Canvas Html with Embed Fonts
A means of encoding a particular truetype font at a particular size into an image and a function to print by extracting characters from the image. Prettier for large type, doesn't kern as well at small sizes. Takes more bytes.
A way of floating text over the canvas so it can be rendered with the standard HTML mechanisms. Nice font handling, but no control over rotation and placement is a little loose.


The author, Jim Studt, can be contacted at Don't write like a spammer if you want any hope of getting through the filters.